The present invention relates to a method of evaluating the reliability or the like of an insulating layer of a semiconductor device, a semiconductor-device-production-process control method using this evaluation method, and a recording medium for causing a computer to execute the evaluation procedure.
With the recent demand for higher density, higher integration and miniaturization of a semiconductor integrated circuit device, an insulating layer, particularly a SiO.sub.2 layer (hereinafter referred to as oxide layer), tends to be reduced in thickness in each of the MOS transistors or MOS capacitors forming the device. On the other hand, since the semiconductor integrated circuit device is not lowered in power voltage so much, the field intensity applied to the oxide layer is increased as the oxide layer is reduced in thickness. Under such circumstances, the time dependent dielectric breakdown (TDDB) of an oxide layer is now regarded as an important issue on which the reliability relies. More specifically, when an electric field is applied to the oxide layer, for example when a voltage is applied across the gate electrode and the channel region of a MOS transistor, or across two electrodes of a MOS capacitor, the oxide layer is broken down at the point of time when a certain period of time has passed from the start of application of the electric field. As a result, the oxide layer looses the electric insulating properties, causing the gate and channel or the upper and lower electrodes sandwiching the oxide layer to be electrically short-circuited. The period of time above-mentioned will be hereinafter referred to as oxide layer lifetime.
Examples of the TDDB include (i) an intrinsic failure which occurs in an originally good oxide layer when a predetermined period of time has passed from the start of application of a voltage and in which the incidence distribution function has a peak, and (ii) an accidental failure which occurs in an originally defective oxide layer earlier than an intrinsic failure. Burn-in is executed for screening an accidental failure of oxide layer of each product. The burn-in condition is determined as follows. First, a field acceleration coefficient of the stress is obtained from the field-intensity-dependency of the oxide layer intrinsic failure periods (lifetimes) obtained through a TDDB test. With the use of the field acceleration coefficient, the burn-in condition is determined as a condition under which an oxide layer accidental failure can be screened within the range where no oxide layer intrinsic failure occurs. As the field acceleration coefficient of the stress for an accidental failure, there is used the field acceleration coefficient measured for the intrinsic failure.
Thus, a time dependent dielectric breakdown test (TDDB test) for extraction of a field acceleration coefficient and estimation of an oxide layer lifetime, is essential in designing and developing a semiconductor integrated circuit device, and tends to be increasingly important.
Examples of the time dependent dielectric breakdown test (TDDB test) include, as shown in the following Table, a constant voltage test in which constant voltages are applied until insulating layers are broken down, a constant electric current test in which constant electric currents are applied until insulating layers are broken down, a ramp voltage test in which a voltage increasing at a constant increasing rate with the passage of time, is applied to the insulating layer, and a ramp (electric current test in which an electric current increasing at a constant increasing rate with the passage of time, is applied to the insulating layer.
TABLE ______________________________________ Test Information Test Type Object Time to be Obtained ______________________________________ Constant voltage Estimation of actual-use long .tau. test lifetime (lifetime) (Absolute evaluation) Constant electric Evaluation of oxide layer long Qbd current test in quality (Breakdown electric (Relative evaluation) charge amount) Ramp voltage test Evaluation of oxide layer short Ebd in defect (Dielectric strength) (Relative evaluation of accidental failure) Ramp electric Evaluation of oxide layer short Qbd current test in quality (Breakdown electric (Relative evaluation) charge amount) ______________________________________
Of the tests above-mentioned, the constant voltage test is to be conducted to estimate an actual-use lifetime .tau.. By this test, an absolute evaluation can be made on the lifetime of the insulating layer. The constant electric current test is to be conducted to obtain an electric charge to breakdown Qbd serving as an index based on which the insulating layer is evaluated in quality. By this test, a relative evaluation can be made on the quality of the insulating layer. The ramp voltage test is to be conducted to obtain the breakdown electric field Ebd serving as an index based on which the insulating layer is evaluated in defect density. By this test, a relative evaluation can be made on the incidence of accidental failure. Likewise the constant electric current test, the ramp electric current test is to be conducted to obtain the electric charge to breakdown Qbd. By this test, a relative evaluation can be made on the quality of the insulating layer. It is noted that when finely checked for waveform, the voltage and electric current in actually conducted ramp voltage and electric current tests, mostly undergo a gradual change and hardly increase perfectly continuously with the passage of time.
With reference to FIG. 9 to FIG. 11, the following description will discuss a conventional constant voltage test for extracting a field acceleration coefficient and estimating an oxide layer lifetime. FIG. 9 is a flow chart illustrating the steps of estimating an oxide layer lifetime through a conventional constant voltage TDDB test. FIG. 10 is a view illustrating the results of measurement of oxide layer lifetime at each stress field intensity, obtained through the conventional constant voltage TDDB test. FIG. 11 is a view illustrating a method of finally determining the oxide layer lifetime in the conventional constant voltage TDDB test.
As shown in FIG. 9, at a step ST51, constant voltages are applied to oxide layers through two conductive layers which sandwich the oxide layers. At a step ST52, there are measured the periods of time (oxide layer lifetimes) between the starts of voltage application and the breakdown of the oxide layers. At a step ST53, the oxide layer lifetimes at the respective field intensities are approximated in the form a function of the field intensities. At a step ST54, a field acceleration coefficient is extracted from the function and the oxide layer lifetime at an optional field intensity is estimated. According to these steps, the stress oxide layer lifetime is estimated.
FIG. 10 is a view illustrating in detail the steps ST51 and ST52. In FIG. 10, the axis of abscissa represents stress time at which an electric field is applied to oxide layers, the right-hand axis of ordinate represents the accumulated failure rate P due to time dependent dielectric breakdown of oxide layers, and the left-hand axis of ordinate represents 1n{-1n(1-p)} calculated from the accumulated failure rate P. Black triangle points, white circle points, white square points and white triangle points respectively represent the accumulated failure rates at the points of time when oxide layers were broken at the stress field intensities Ea, Eb, Ec, Ed. Straight lines ap, bp, cp, dp connecting points of the same types, represent the time dependencies of accumulated failure rates at the respective stress field intensities Ea, Eb, Ec, Ed. Respective stress times Ta, Tb, Tc, Td at the intersecting points where the respective straight lines intersect the long and short dash line, are the measured values of 50% accumulated failure time due to time dependent dielectric breakdown of oxide layers. As shown in FIG. 10, plotting the accumulated failure rates P on a graph having a logarithm of time on the axis of abscissa and 1n{-1n(1-p)} on the axis of ordinate, is generally called a Weibull plotting. Generally, when the failure probability follows a Weibull distribution, the results of Weibull plotting are represented in the form of a straight line. When it is desired to express, on a graph, the accumulated failure rates of time dependent dielectric breakdown of oxide layers with respect to stress times, a Weibull plotting method is widely used.
FIG. 11 is views illustrating in detail the steps ST53 and ST54. In FIG. 11, the axis of abscissa represents the field intensity E applied to oxide layers and the axis of ordinate represents the oxide layer lifetime T. In FIG. 11, Ta, Tb, Tc, Td refer to the measured values of 50% accumulated failure time, EA, EB, EC, ED refer to stress field intensities, a straight line T(E) represents an estimation line of oxide layer lifetime, Emax is the maximum field intensity applied to the oxide layer when the oxide layer is actually used, and .tau.ES is an estimated value of oxide layer lifetime when the oxide layer is actually used.
The following description will discuss a specific process for executing the steps above-mentioned. To estimate an oxide layer lifetime, there are prepared a number of MOS capacitors respectively having, as capacitor insulating layers, oxide layers identical in shape, sizes and production process with one another. These MOS capacitors are divided into a plurality of groups. There is applied, to each of the groups, each of the constant stress field intensities EA, EB, EC, ED which are higher than the maximum field intensity Emax which is shown in FIG. 11 and which is to be applied to each oxide layer at a normal operating condition. By such stress application, oxide layers of each group experience a time dependent dielectric breakdown and the number of failed oxide layers is increased with the passage of time.
The periods of time (stress periods of time) between the starts of application of stress field intensities and the breakdowns of oxide layers, and the accumulated failure rates; at the points of time of such breakdowns, are plotted in a Weibull manner as shown in FIG. 10. Empirically, the failure rates of time dependent dielectric breakdown of oxide layers follow a Weibull distribution. Accordingly, each Weibull plotting of time-dependent accumulated failure rates is generally expressed in the form of a straight line. Based on these data, there are obtained regression lines ap, bp, cp, dp for the accumulated failure rates at the points of time when oxide layers are broken down at stress field intensities, and there are obtained the actually measured values Ta, Tb, Tc, Td of 50% accumulated failure time, i.e., the periods of time during which the accumulated failure rates reach 50% at the respective stress field intensities EA, EB, EC, ED.
With the 50% accumulated failure times regarded as oxide layer lifetimes, there are plotted, in a semi-logarithmic graph in FIG. 11, the actually measured values Ta, Tb, Tc, Td of 50% accumulated failure time on the axis of ordinate, and the field intensities EA, EB, EC, ED applied to the oxide layers on the axis; of abscissa. Empirically, the actually measured values Ta, Tb, Tc, Td of 50% accumulated failure time can be plotted on a straight line. Based on this, the actually measured values Ta, Tb, Tc, Td, of 50% accumulated failure time, i.e., the oxide layer lifetimes T, are approximated in the form of a function of field intensity E, e.g., in the form of a regression line T(E). The slope of the regression line T(E) is extracted as a stress field acceleration coefficient .beta. (decades/MV/cm). Using this regression line T(E), there can be obtained the estimated value of oxide layer lifetime at an optional field intensity. Then, there is obtained the lifetime estimated value .tau.ES of oxide layer at a normal operating condition, i.e., at the maximum value Emax of field intensity actually applied to the oxide layer.
To evaluate, in a short period of time, the reliability of oxide layers with the use of MOS capacitors on a wafer for which the diffusion step has been finished in a production process, a test is to be conducted, on a wafer level, on a plurality of MOS capacitors each having, as a capacitor insulating layer, an oxide layer to be evaluated. Using an automatic prober, the test is to be conducted on the MCS capacitors on a wafer with the measurement points successively probed.
Of the conventional methods of estimating the oxide layer lifetime, the constant voltage test has the following problems.
In development of a semiconductor device production process, a process condition is frequently changed. A change in process condition likely changes the field acceleration coefficient and the oxide layer lifetime. It is therefore required to frequently conduct an extraction of field acceleration coefficient and an estimation of oxide layer lifetime.
In a mass production at a factory, there are instances where a process condition is changed due to trouble of a production machine or the like such that the wafers for which the diffusion step has been finished, vary in oxide layer lifetime. In the worst case, there is produced a wafer having an oxide layer of which lifetime is too short to satisfy the standards for assuring the reliability. If such an abnormality occurs, to prevent a product low in reliability from being delivered from the factory to the market, it is required to rapidly and accurately estimate the lifetimes of the oxide layers.
More specifically, it is required to obtain, in a short period of time, the field acceleration coefficient and the estimated oxide layer lifetime for each of the wafers for which the diffusion step has been finished.
When conducting a test of lifetime estimation on a wafer level according to any of the conventional oxide layer lifetime estimating methods above-mentioned, an automatic prober is used for measurement in which the MOS capacitors on a wafer are successively probed. Accordingly, the number of MOS capacitors which can simultaneously be subjected to the test, is limited to one or several which can simultaneously be electrically connected to the probe needles of a probe card. Thus, when a number of stress field intensities are applied to a number of MOS capacitors to measure the accumulated failure rates, this takes much time. In particular, this is remarkable in measurement at low stress field intensities and therefore constitutes a great hindrance to the extraction of field acceleration coefficients and estimation of oxide layer lifetimes of all the wafers.
To shorten the period of time for estimation of oxide layer lifetime, the number of MOS capacitors to be used can be reduced or the stress field intensities to be used in the test can be increased.
However, it is required to obtain the accumulated failure rates using a variety of different stress field intensities. Accordingly, when the number of MOS capacitors to be used in the measurement is reduced, the number of MOS capacitors assigned for each stress field intensity condition is further reduced. Even though it is supposed that the MOS capacitors on a single wafer are equivalent to one another, the MOS capacitors are actually different in characteristics from one another. The accumulated failure rate at each stress field intensity is obtained from different MOS capacitors. Therefore, as the number of MOS capacitors assigned to each stress field intensity is smaller, the variations in characteristics of the MOS capacitors appear in the form of displacement of the distribution of accumulated failure rate at each stress field intensity. As a result, the variations in characteristics of the MOS capacitors appear in the form of displacement of oxide layer lifetime at each stress field intensity. This finally increases the errors in the field acceleration coefficient and the estimated value of oxide layer lifetime. In view of assurance of reliability, it is therefore not recommended to estimate the field acceleration coefficients and oxide layer lifetimes of all the wafers with MOS capacitors to be used for measurement being reduced in number in order to shorten the whole measuring period of time.
On the other hand, when the stress field intensities are increased, the following problems are encountered. First, the measuring device becomes poor in time precision. Further, the application of high electric fields generates a large amount of heat, causing the oxide layers to be unforeseeably increased in temperature. This shortens the periods of time between the starts of application of electric fields and the time-dependent dielectric breakdowns of the oxide layers, as compared with the periods of time taken with the use of the normal temperatures and field intensities. This produces great errors in the extraction of field acceleration coefficient and lifetime estimation. It is therefore not recommended to extract the field acceleration coefficients and to estimate the oxide layer lifetimes of all the wafers with the stress field intensities increased in order to shorten the whole measuring period of time.
Now, studies are also made for other TDDB tests than the constant voltage test. The constant electric current test can be shorter in test period of time than the constant voltage test, but still takes much time and is disadvantageous in that only relative evaluation of oxide layer in quality can be made. The ramp voltage test is shorter in test period of time, but is not fit for the object of evaluating the normal insulating layer lifetime. That is, there is obtained, by this test, information for estimating the incidence of accidental failure due to defect. The ramp electric current test is short in test period of time, but is also disadvantageous likewise the constant electric current test.
After all, according to any of the conventional test methods above-mentioned, there cannot be obtained, in a practically short period of time, a field acceleration coefficient and an oxide layer lifetime estimated value which are practically highly precise. In particular, it is not possible to extract the field acceleration coefficients of all the wafers and to estimate the oxide layer lifetimes of all the wafers.